78 ELECTRIC LIGHTING. 



lower part are traversed by currents in precisely the opposite 

 direction; but as at the two points where the junction of 

 these two opposite currents take place the circuit has a deri- 

 vation by means of the rubbing pieces F and F', connected 

 with the external circuit, they flow through this derivation 

 united in quantity. 



Let us now examine what happens from the motion of 

 these coils with regard to the magnetism developed in the 

 ring, and in order to fix our ideas, we shall suppose that only 

 one annular electro-magnet turning between the two poles 

 N s of an iron horseshoe magnet is magnetized in such a 

 manner that the system appears to be constituted of two 

 semicircular magnets joined to each other by their similar 

 poles, whence it follows that on the equatorial line, that is to 

 say, on the diameter perpendicular to the line joining the two 

 poles of the inducing magnet, there exist two neutral regions, 

 and it follows also that the magnetic coil is wound from left 

 to right in one side of the ring, and from right to left in 

 the other side. Now let us see what takes place when coils 

 of a short length, such as those previously referred to, are 

 made to move before such a magnetic system, and let us 

 follow, as before, the course of the short coil E, supposing 

 the iron ring to be stationary. In leaving the neutral line 

 on the right to move towards the point s, the coil will be 

 receding from the resultant of all the magnetic spires of the 

 right part of the ring corresponding with its neutral line, and 

 there will be developed in it a direct current, as in the case 

 in which, without a magnet ring, it is made to approach the 

 pole s. When it has passed the upper part of the ring, it 

 will approach the resultant of the magnetic spires of the left 

 part of that ring, and give rise to an inverse current ; but on 

 account of the different direction of its winding, this current 

 will traverse the circuit in the same direction as the first 

 current, and it will be only at the neutral line towards the 

 left that these effects will change. The coil will then be 

 found to undergo the same reactions as those we have before 



